The present disclosure generally relates to dielectric films in semiconductor devices, and more particularly, to ultraviolet (UV) curing processes for low-k dielectric films. More particularly, embodiments of the disclosure relate to a process for improving the electrical, chemical, and mechanical properties of silicon based materials that are utilized as integrated circuit (IC) dielectrics.
New materials with low dielectric constants (known in the art as “low-k dielectrics”) are being investigated for their potential use as insulators in semiconductor chip designs. A low dielectric constant material aids in enabling further reductions in the integrated circuit feature dimensions. In conventional IC processing, SiO2 is used as a basis for the dielectric material resulting in a dielectric constant of about 3.9. Because of this, the term “low-k dielectric” generally refers to materials having a dielectric constant less than SiO2, that is, a dielectric constant generally less than 3.9. More typically, for the advanced design rules, the dielectric constant is less than 3.0, and oftentimes less than 2.5.
The substance with the lowest dielectric constant is air (k=1.0). Therefore, porous dielectrics are very promising candidates since they have the potential to provide very low dielectric constants. The films are generally deposited or formed using a spin-on process or a chemical vapor deposition (CVD) process. Unfortunately, however, porous low-k dielectrics typically have the problem of insufficient mechanical strength and deposition temperatures of these materials can exceed allowable thermal budgets, which properties are needed to withstand the stresses of back end of line processing (BEOL). For example, after the formation of the low-k film, a cure process is generally performed to complete the formation of chemical bonds, outgas residual components, form pores, and/or reduce the dielectric constant in the film. This curing process is commonly performed in a batch mode using a furnace or in a single wafer mode using a hotplate. In either case, the conventional cure process undesirably subjects the wafer to an elevated temperature for an extended period of time (e.g., in excess of one hour to several hours and at a temperature in greater than about 300° C.). In addition to the problems related to the low-k dielectric's thermal and mechanical properties, the so-cured dielectric materials generally have relatively poor wet etching resistance, an area of concern where improvement is desired. Moreover, the time and equipment required to perform additional processes can undesirably add cost to a manufacturing process.
In addition to thermally curing the low k dielectric material, low k materials are oftentimes exposed to UV radiation in a controlled environment so as to remove porogens, and water; stabilize the k value of the low k dielectric material; and provide improved electrical and mechanical properties. Current processes of record uses the action of UV light in a controlled inert atmosphere such as nitrogen (N2) or in a vacuum. The effects of the cure process on the low k material have recently been analyzed before and after UV exposure in the controlled inert atmosphere using FTIR. As will be discussed in greater detail below, it has been discovered that the inert atmosphere results in the formation of undesirable sub-oxidized species, i.e., silicon atoms with less than 4 Si—O bonds. The consequence of the sub-oxidized silicon groups (also referred to herein as sub-oxidized silicon species) with unbonded sites (i.e., coordination number is less than 4) includes, among others, a decrease in film electrical and chemical stability and a decrease in film integrity. These trends in mechanical properties are undesirable.
Accordingly, there remains a need for a curing process that provide silicon based low-k materials with improved electrical, mechanical and chemical properties, including improved elastic modulus and material hardness, without compromising or deteriorating its electrical properties.